Thursday, June 28, 2007


Silicone enhanced RepRap

I have got the paste extruder working on RepRap. It consists of a 500 ml fizzy drink bottle containing a balloon that in turn contains the paste. You pressurize the bottle using a bicycle pump, and the flow is controlled by a latching solenoid that clamps onto a soft silicone tube on the way to the nozzle. The clamp is the red rectangular piece hovering over the tube in the picture below (the solenoid is behind the green support). When the red bit moves backwards the flow is shut off.

Here is a close-up of it laying down a test square. The material is PolyDiMethylSiloxane (PDMS - silicone bathroom sealant). I had the flow set too high and the movement speed too slow, so it's flooding a bit. But that should be easy to fix.

The controller electronics are really simple - just the standard RepRap controller with slightly modified firmware (which I have just posted in the software repository as extruder_1).

The silicone doesn't set if you leave it in the device overnight, except for the bit actually in the silicone tube that forms the valve. I suspect that this is caused by the fact that the materials are the same, and so a little of the catalyst that caused the tube to polymerise when it was originally made is leeching out and setting the working material as well. If I am right, to fix this we just need a soft tube of another material.

When the valve closes, it deposits a splodge of goo as it displaces some material. The tube I used for the valve is 3mm internal diameter, which it doesn't need to be. I'll try and find some of 1mm I.D. That should cut down the closure volume, and hence the problem, dramatically.

This means that we should be able to work with just about any material that is a paste at room temperature. Tomorrow I'll use up another balloon (no expense spared) and try Polyfilla wall-crack filler paste.

Labels: ,

Great stuff, Adrian! So you are using silicone sealant instead of Polyfilla now? Were there problems with Polyfilla that made you want to change or are you just trying different materials?

With that pressurised plastic kooldrank bottle I'm wondering why you didn't just design it so that you could screw in a standard silicone sealant tube into the extruder. I'd think that that would be the easiest.

I was trying to estimate the distance between your support material extruder and your Mk II extruder and got something like 50 mm. Is that about right?
To my knowledge, what causes PDMS to set is not a catalyst, but a reactant.

The reaction, as I understand it, is:

(CH3)2SiR2 + H2O = 2 HR + [(CH3)2SiO]

Where R is, for bathroom sealant, acetate and, for your tube, probably chlorine. I think the sealant is sold partially polymerized, but the reaction is the same, and a catalyst wouldn't provide the necessary oxygen.

Water vapor diffuses through silicone, which is what allows the interior of a bead of caulk to set; I believe it's also diffusing through your tube, and through the plug of set material at the end, as well.

What you might consider is keeping the extruder in a dessicator when not in use. Chemical supply companies sell nice absorbent granules that change color when they're damp, and can be baked dry very easily.
Uh, but vinyl tubing might help; I think it forms a fairly good barrier to water. Presumably, the air in the drink bottle dries out before much of the material in the balloon sets; I know polyisoprene is water-permeable. You are keeping it in the bottle, right?

The best rubber where permeability is concerned is polybutediene (aka butyl rubber). I'm not sure you can get tubes made from it, but they do make gloves.
Goshdarnit, Adrian. That's looking good. I was wondering how the two heads would fit together, and it's good to see them doing so snugly.

Vik :v)
I just used silicone first because it's fairly easy to handle and I happened to have some. I'll try the Polyfilla later today.

It is using the standard extruder nozzle arrangement more or less. I modified it a bit for this prototype, but a production version would use it unmodified.

50 mm is about right. I'll do a test object which can be measured to put the actual numbers in as offsets in the properties file.

Joel's PDMS reaction info is most useful. I was going on the fact that we have a drum of the stuff for moulding that we have to mix 10:1 with a hardner, which I had ignorantly presumed was a polymerisation catalyst. But it is certainly the case that it is water vapour that makes this stuff set (and the acetate gives the usual vinegar bathroom sealant smell). I did know that water can diffuse through the silicone tube. I think if I can find a soft impermeable tube that will fix the problem.
Post a Comment

<< Home

This page is powered by Blogger. Isn't yours?

Subscribe to
Posts [Atom]